Single stage down converting heterodyne circuits are sometimes used in applications, where it is desirable to remove the effect of unwanted frequencies. For example, in modern semiconductor fabrication processes that utilize plasma chambers for processing wafers, outputs from multiple RF generators, operating at different frequencies, are injected into a single plasma chamber. In this environment, the mixed frequencies need to be separated in the power detection circuitry of each RF generator so that the power output from each RF generator may be controlled based solely upon characteristics at the operational frequency of the respective RF generator, and not upon characteristics at the operational frequency of any other RF generator. A heterodyne control circuit may be used to achieve this goal.
In the prior art of semiconductor fabrication processes that employ multiple RF generators to inject power into a plasma chamber, a heterodyne circuit is often used to separate the operational frequencies. Signals at the output of each RF generator are sensed by an output sensor, filtered through a heterodyne circuit to isolate the desired signal, which is then provided to the power detection circuit, the output of which is used for feedback control of the respective RF generator. The signals sensed by the output sensor, for each RF generator contain each of the signals injected into the plasma chamber, along with mixed signals and noise signals, and these sensed signals may be any characteristic of the power signals injected into the plasma chamber, such as voltage, current, or power.
By way of a specific example, in the case of a single plasma chamber that is injected with power from two RF generators, the first RF generator operating at 60 MHz, and the second RF generator operating at 2 MHz, the resulting mix of frequencies produced by this system may include signals at 60 MHz, 2 MHz, 58 MHz, and 62 MHz, with the latter two frequencies being byproducts of mixing the first two frequencies within the plasma chamber. The output sensor for each of the first and second RF generators will sense signals at 60 MHz, 2 MHz, 58 MHz, 62 MHz, along with any signals resulting from noise. The heterodyne circuit down converts the sensed signals and enables the 60 MHz signal to be more easily separated from the 58 MHz and 62 MHz signals. Without the heterodyne circuit, separation of the close frequencies would be otherwise difficult. The heterodyne circuit may down convert the 60 MHz signal by mixing with an oscillator signal having a frequency that is offset from the 60 MHz signal by a predetermined amount, such as less than 1000 kHz. The resulting mixed signal is a function of the detected 60 MHz signal, and with proper filtering, the mixed signal can be used to perform power detection without being negatively impacted by the 58 MHz and 62 MHz signals.
The power detection circuit receives the mixed and filtered signal from the heterodyne circuit. Typical power detection circuits for an RF signal generally employ a peak detector, and the typical peak detector includes one or more hold capacitors that are charged by the input RF signal. The hold voltage from the one or more hold capacitors is then converted into a DC signal, and the DC signal is used by the control circuitry to determine the output of the RF generator and to thereby control the RF generator.
In some plasma applications, it is desirable to pulse the output power of one or more of the RF generators at a predetermined frequency in the MHz range. When the power during each pulse needs to be accurately monitored, the typical heterodyne and detecting circuits fall short. This is because the typical hold capacitor for a signal having a frequency of 1000 kHz or less is incapable of charging fast enough to measure the peak value of pulses in the MHz range, so that the measurement of power detection accuracy deteriorates. Under this circumstance, the peak value of the detected signal becomes dependent upon the peak voltage of the hold capacitor, which in turn can vary with the tolerance of the hold capacitor. This can result in the peak value of the detected signal varying by 7%-8%. Such a variance in the error signal results in errors in controlling the output of the RF generator, which then directly affects the processes performed within the plasma chamber. Therefore, a new method for processing and detecting RF signals for purposes of providing feedback to an RF generator is desirable.